Cold sealable cohesive terpolymers

ABSTRACT

A cold sealable emulsion polymerized terpolymer cohesive formulation which has low surface tack characteristics, prepared from the emulsion polymerization of a first monomer selected from the group consisting of ethyl acrylate, hexyl acrylate, isooctyl acrylate, butyl acrylate, methyl acrylate and vinylidene chloride, a second monomer selected from the group consisting of vinyl acetate, methyl methacrylate, styrene, ethyl methacrylate and isobutyl methacrylate and a third monomer selected from the group consisting of methacrylic acid, acrylic acid, itaconic acid, sulfoethyl methacrylate and malic acid with the proviso that each of said first, second and third monomers are different.

This is a division of copending application Ser. No. 187,303, filed on Apr. 28, 1988 and now U.S. Pat. No. 4,898,787, which is a continuation-in-part of copending U.S. patent application Ser. No. 069,049, filed by the inventors herein on Jul. 2, 1987 now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a cold sealable emulsion polymer cohesive composition which is low in surface tack, and to cold sealable articles, particularly packaging film treated with the same.

2. Background of the Related Art

In the past, in the preparation of films useful for packaging purposes, heat sealable coatings, such as acrylic coatings, were coated on one side of the film substrate and another heat sealable coating, such as polyvinylidene chloride (PVDC) was coated on the other side. The acrylic coated side was generally the outside of the web, the side in direct contact with the hot sealer surfaces, where good hot slip and jaw release characteristics are required. The PVDC coating was usually on the inside of the web and provided the high seal strength, good hot tack characteristics and barrier properties required for such packaging. These heat sealable coatings have glass transition temperatures ("Tg"s) which are higher than room temperature. Such a coated film is disclosed in U.S. Pat. No. 4,403,464.

Similarly, U.S. Pat. No. 4,456,741 discloses heat sealable terpolymer compositions useful as pressure-sensitive adhesives for application with, for example, backing materials including paper, polyester film and foamed polymers. The terpolymer heat sealable pressure-sensitive adhesive composition comprises butyl acrylate, N-vinyl-2-pyrrolidinone and styrene. Other heat sealable coatings are disclosed in U.S. Pat. No. 3,696,082; and East German Patent No. DD-146,604.

In packaging products which are sensitive to heat, such as candies, chocolates, ice cream and the like, in plastic film or paper packages, the use of heated elements must be avoided in order to prevent melting of the products. Therefore, the use of heat sealable coatings to seal the packages has presented serious difficulties in packaging the product, often requiring the isolation of the product from the heated elements. Accordingly, cold sealable pressure-sensitive adhesives were developed which did not require the use of a heated element to seal the packages. Unfortunately, these adhesives had high surface tack characteristics which made them adhere to uncoated surfaces of the packaging film, making these adhesives difficult to use due to the resulting blocking (i.e. sticking) of the film.

Findley 207-939, a polyisoprene adhesive manufactured by Findley Adhesive, Inc., is a cold sealable pressure-sensitive adhesive coating which exhibits good crimp seal strength on oriented polypropylene film and has a Tg of -1.6° C. This adhesive has a high surface tack which often results in blocking of the packaging film.

Another such pressure-sensitive adhesive composition is disclosed in U.S. Pat. No. 4,500,683. The pressure sensitive adhesives of this patent contain as a polymer component an addition polymerization polymer of an acrylate or methacrylate polymer and one or more ethylenically unsaturated monomers capable of forming homo- or copolymers having a glass transition temperature of at least 273° k. This composition forms a viscous adhesive composition which is tacky at room temperature, thus presenting the blocking problems when used on packaging films.

Thus, the related art has disclosed heat sealable coatings and pressure-sensitive, tacky, cold sealable adhesives useful in the packaging art. The related art has not, however, disclosed or provided a cold sealable, pressure-sensitive cohesive, rather than adhesive formulation which similarly to the above described heat sealable, pressure-sensitive coatings, has low surface tack and is cohesive only when placed under pressure in contact with other cold sealable cohesive coated surfaces.

Accordingly, it is an object of the present invention to provide a coal sealable, emulsion polymerized terpolymer cohesive formulation which is cohesive to similarly coated film or paper substrates, rather than being tacky and adherent to non-coated surfaces.

Another object of the present invention is to provide a cold sealable cohesive formulation having the unique improved combination of properties, e.g., low surface tack, good coating uniformity, high seal strength and wetting ability, on substrates such as plastic and paper film.

Still a further object of the invention is to provide a cold sealable, emulsion polymer cohesive formulation which, when coated on a film or paper substrate, imparts high cold seal strength, i.e., bonding under only pressure, even at low coating weight and also improved surface properties, e.g., low surface tack and excellent coating uniformity.

Still another object of the present invention is to provide improved packaging film or paper coated with a cold sealable cohesive formulation having a smooth, non-tacky surface which will not block at room temperature.

A further object of the invention is to provide improved coated packaging film having high cold seal strength.

SUMMARY OF THE INVENTION

These and other objects are achieved according to the present invention by providing an emulsion polymer, cold sealable, cohesive composition prepared from the emulsion polymerization of a first monomer selected from the group consisting of ethyl acrylate, hexyl acrylate, isooctyl acrylate, butyl acrylate, methyl acrylate, vinylidene chloride and mixtures thereof; a second monomer selected from the group consisting of vinyl acetate, methyl methacrylate, styrene, ethyl methacrylate, isobutyl methacrylate and mixtures thereof; and, a third monomer selected from the group consisting of methacrylic acid, acrylic acid, itaconic acid, sulfoethyl methacrylate, maleic acid and mixtures thereof with the proviso that each of said first, second and third monomers are different.

The resulting latex formulation adheres, when wet, to packaging film substrates, such as oriented polypropylene ("OPP") or paper. Once solidified by drying, the coated surface is cohesive at room temperature and under pressure only to other similarly coated surfaces. The coated surface does not adhere to non-coated film surfaces, and thus presents a smooth, non-tacky surface which will not block or stick under normal operating conditions.

DETAILED DESCRIPTION OF THE INVENTION

The emulsion polymerized cold sealable polymer formulations of the present invention are prepared by a two-stage emulsion polymerization process. The first stage of polymerization, which may be referred to as the seed latex, is carried out, for example with about 5% to about 30% by weight of the total monomer feed, preferably about 10% of the total monomer feed. The second stage of polymerization is carried out with the remaining 90% by weight of the monomer feed. Each stage of the polymerization is carried out in the presence of water, an emulsifying agent and a free radical catalyst. Typical free radical catalysts include hydrogen peroxide, ammonium or potassium persulfate or a redox type, such as mixtures of persulfates with alkali metal bisulfites, thiosulfates or hydrosulfites. Generally, the total amount of catalyst employed is in the range of from about 0.5% by weight to about 5% by weight based on the total weight of the monomer. Each stage of the emulsion polymerization is typically carried out in the presence of one or more anionic, nonionic or cationic emulsifiers such as, for example, the alkyl carboxylic acid salts, phosphoric acid salts, the alkyl sulfate salts, the alkyl sulfonate salts, the alkyl aryl ether alcohols and the alkyl aryl polyether sulfate salts. Generally, the total amount of emulsifier employed is from about 0.01 to about 1.0 percent by weight based on the total amount of water.

A chain transfer agent, such as, isooctyl thioglycolate, may also be employed in the emulsion polymerization process herein. Generally, the chain transfer agent is utilized in amounts ranging from about 0.1 to about 0.9% by weight based on the weight of total monomer.

In general, the first stage of the polymerization is carried out at a temperature of from about 40° to about 90° C, preferably about 80° C., at a pressure in the range of from about 0 to about 20 psig, preferably about 10 psig. This stage may or may not be carried out in the presence of an insert gas, such as nitrogen. The first stage polymerization is generally carried out for a time of from about 0.5 to about 3 hours, preferably about 1 hour.

The second stage polymerization is generally performed under the same temperature and pressure conditions as in the first stage. Typically, the remaining amount of monomer feed is gradually fed to the second stage over a period of from about 2 to about 5 hours. The total reaction time of the second stage, however, is usually from about 4 to about 6 hours.

In accordance with the present invention, the terpolymers prepared from the above described two stage emulsion polymerization process are prepared from a monomer feed comprised of a first monomer selected from the group consisting of ethyl acrylate, hexyl acrylate, isooctyl acrylate, butyl acrylate, methyl acrylate, vinylidene chloride and mixtures thereof; a second monomer selected from the group consisting of vinyl acetate, methyl methacrylate, styrene, ethyl methacrylate, isobutyl methacrylate and mixtures thereof; and, a third monomer selected from the group consisting of methacrylic acid, acrylic acid, itaconic acid, sulfoethyl methacrylate, maleic acid and mixtures thereof.

The glass transition temperatures of homopolymers from these monomeric constituents are listed in Table 1.

                  TABLE 1                                                          ______________________________________                                                         Homopolymer Tg's                                                               Tg (°C.)                                                ______________________________________                                         First Monomer Group;                                                           Ethyl acrylate    -22                                                          Hexyl acrylate    -57                                                          Isopropyl acrylate                                                                                -3                                                          Butyl acrylate    -54                                                          Methyl acrylate     8                                                          Vinylidene chloride                                                                              -17                                                          Second Monomer Group;                                                          Vinyl acetate      32                                                          Methyl methacrylate                                                                               105                                                         Styrene            100                                                         Ethyl methacrylate                                                                                65                                                          Isobutyl methacrylate                                                                             48                                                          Third Monomer Group;                                                           Acrylic acid       106                                                         Methacrylic acid   185                                                         Itaconic acid     --                                                           Sulfoethyl methacrylate                                                                          --                                                           Maleic acid       --                                                           ______________________________________                                    

In general, the terpolymers of the present invention are prepared from and contain about 40 to about 75 weight percent, preferably 60 to 70 weight percent, of the aforementioned first monomer, such as ethyl acrylate, from about 15 to about 50 weight percent, preferably 25 to 35 weight percent, of the aforementioned second monomer, such as vinyl acetate, and from about 0.1 to about 10 weight percent, preferably 1 to 5 weight percent, of the aforementioned third monomer, such as methacrylic acid, all ratios are based on the total monomer weight. Optionally if desired a small amount, preferably less than 0.5% by weight, of a multifunctional monomer, such as, for example, ethyleneglycol dimethylacrylate, may be added to either stage of the emulsion polymerization process, preferably the second stage, to provide some crosslinking and increase the sealing strength (crimp strength) of the polymer without affecting the glass transition temperature ("Tg") and the tackiness of the terpolymer.

The emulsion polymers of the present invention have an average molecular weight of from about 20,000 to about 150,000, preferably from about 50,000 to about 90,000 as determined by gel permeation chromatography (GPC) and have a Tg of from about -15° to about 15° C., preferably from about -10° C. to about 1.0° C. The diameter of the terpolymer particles are generally in the range of from about 0.05 to about 0.3 microns.

The glass transition temperature ("Tg") of the polymer formulation is inversely related to the tackiness of the coating. So that when the Tg of the polymer decreases, the tackiness increases. Tg of the terpolymer is related to the ratios of the weight fractions of the monomeric components and the Tg's of these components, so that when a terpolymer is being analyzed, ##EQU1## where: T_(g) =the Tg of the terpolymer;

T_(g1) =the Tg of the first monomer;

T_(g2) =the Tg of the second monomer;

T_(g3) =the Tg of the third monomer;

W_(f1) =the weight fraction of the first monomer;

W_(f2) =the weight fraction of the second monomer; and,

W_(f3) =the weight fraction of the third monomer.

Thus, Table 2 examines the theoretically calculated values for Tg, comparative surface tack ("ST") and crimp seal strength ("Cs") of OPP film coated with one preferred combination of monomeric components. The first monomeric component being ethyl acrylate ("EA"), the second being vinyl acetate ("VAC"), and the third being methacrylic acid ("MAA"). In addition, Table 2 compares the theoretically calculated properties of an OPP film coated with this preferred terpolymer composition falling within the preferred range of proportions of monomeric components, to the theoretically calculated values for an OPP film coated with the terpolymer having various other proportions of those monomeric components falling outside of the preferred range.

                                      TABLE 2                                      __________________________________________________________________________                          Properties                                                Component Composition                                                                               (Theoretically calculated values)                         % by Weight          Tg         CS                                             EA         VAC  MAA  (°C.)                                                                         ST   (g/in)                                         __________________________________________________________________________     Preferred                                                                            60-70                                                                               25-35                                                                                1-5 -10 to 10                                                                            very low                                                                            >438 (high)                                    range 67   30    3    -4.2 low  high                                                 67   32    1    -5.6 low  high                                                 70   27    3    -5.6 low  slightly less                                  Acceptable                                                                           40-75                                                                               15-50                                                                                .1-10                                                                              -15 to 15                                                 range 60   37    3    -0.5 low  less                                                 60   30   10     5.4 very low                                                                            less                                                 67   29.9  0.1   0.7 low  low                                                  67   29.99                                                                                0.01                                                                                 0.8 low  low                                                  75   15   10    -2.7 low  high                                           Below 30   67    3    16.2 very low                                                                            non-sealable                                   acceptable                                                                           20   77    3    22.3 very low                                                                            non-sealable                                   range 30   60   10    22.6 very low                                                                            non-sealable                                         80   17    3   -10.5 tacky                                                                               barely sealable                                      80   10   10    -5.3 less tacky                                                                          high                                           __________________________________________________________________________

The resulting latex formulation adheres, when wet, to packaging film substrates, such as oriented polypropylene ("OPP") or paper. Once solidified by drying, the coated surface does not adhere to non-coated film surfaces, and thus presents a smooth, non-tacky surface which will not block or stick under normal operating conditions. However, if similarly coated surfaces are placed in contact with each other and under pressure, at about room temperature or below, then the coated surfaces become cohesive forming a strong bond between each other.

Thus, the emulsion polymer formulations of the present invention are very useful in imparting high cold seal bond strength to packaging paper or packaging film substrates, particularly polyolefin film, such as polypropylene film substrates. Substrates which are particularly contemplated for use with the cold sealable polymer formulations of the present invention include cast or oriented polypropylene, coextruded films, nylon, white opaque film, cellophane, paper, polyesters, high density polyethylene and linear low density polyethylene. It is also contemplated herein that the substrates may be uncoated or coated, such as with polyvinylidene chloride or primer and/or may be pre-treated, such as with corona discharge, flame or plasma. It has been found herein, and as will be substantiated by the data hereinafter, that the cohesive terpolymer formulations of the present invention, when coated on packaging film, such as polyolefin film, impart high cold seal strength, (bonding using only pressure) against the same coated side, with good coating uniformity and low tack, even at low coating weights.

Commonly known techniques can be employed to apply the emulsion polymer formulation of the present invention to the film or paper substrate. For example, when impregnating or saturating the substrate, it may be dipped or sprayed. If the substrate is coated, this may be accomplished by dipping, spraying or by employing a roller, spreading knife, brush or the like. Generally, the emulsion polymer formulation is applied in an amount of from about 1 lb./3000 sq. ft. to about 5 lbs./3000 sq. ft. to the film substrate.

The emulsion polymerization formulation of the present invention may be compounded with, or have mixed therein, other known ingredients or stabilizers, antifoaming agents, dying adjuvants, pigments, waxes, corn starch, silica, talc and the like or other compounding aids to control surface tack and other surface properties. Thickeners or bodying agents may be added to the polymers so as to control the viscosity of the polymer and thereby achieve the proper flow properties for the particular application desired.

The following examples are illustrative of the invention.

EXAMPLE 1

This example illustrates the preparation of an emulsion polymerized, cold sealable, terpolymer formulation within the scope of the invention.

Using a semi-continuous batch process, a seed latex was prepared by continuously adding and metering 10% by weight of premixed total monomer feed to a reactor containing 100 parts by weight water, 0.05 parts by weight 10% phosphoric sodium salt solution (sodium Gafac, GAF Corporation), 0.025 parts by weight 10% ammonium persulfate solution and 0.004 parts by weight isooctyl thioglycolate, as a chain transfer agent, (all parts by weight based on 100 parts by weight water). The premixed total monomer feed is comprised of 67 parts by weight ethyl acrylate, 30 parts by weight vinyl acetate and 3 parts by weight methacrylic acid, based on 100 parts by weight total monomer. Then, 10% by weight of this premixed monomer was added to the reactor over a period of 30 minutes. The emulsion polymerization reaction was maintained isothermally at 80°±2° C. and the pressure was maintained at about 10 psig with inert nitrogen gas. Sufficient agitation was used to uniformly disperse the monomers in the reactor. The reactor batch was held for about 15 minutes after the addition of 10% premixed monomers.

The second stage of polymerization was subsequently carried out by the same semi-continuous batch process wherein the residual premixed monomer (i.e., remaining 90% by weight of the total monomer feed) was added. Prior to commencing the second stage, 4.9 additional parts by weight of 10% phosphoric sodium salt solution (sodium Gafac, GAF Corporation) was added to the reactor. The second stage of monomer feeding was started when the polymerization temperature was raised to 80° C. The monomer metering cycle was 3 hours for the second stage of polymerization. The temperature and pressure were the same as the first stage. After completing the addition of all of the monomer feed, the reactor batch was held for 30 minutes.

For latex stability, a small additional amount of 10% phosphoric sodium salt solution and aqueous ammonia solution were added to the reactor. The latex was subsequently cooled and filtered through a 25 micron filter. The latex stability of the resultant EA/VAC/MAA terpolymer was excellent. The terpolymer has the structure: ##STR1##

The properties of the terpolymer latex prepared by Example 1 are summarized in Table 3 below:

                  TABLE 3                                                          ______________________________________                                         Characteristics of EA/VAC/MAA Terpolymer Latex                                                   Latex                                                        ______________________________________                                         Total Solids (%)    43.0                                                       Particle Size (nm)  102                                                        pH                  6.5                                                        Surface Tension (dynes/cm)                                                                         48.3                                                       Mw × 10.sup.3 73                                                         Mn × 10.sup.3 35                                                         Tg (°C.) (measured)                                                                         4.1                                                        ______________________________________                                    

EXAMPLE 2

An ethyl acrylate-vinyl acetate-methacrylic acid terpolymer latex prepared in accordance with the two-stage emulsion polymerization process of the present invention was pattern applied by roller to the sealable portions of one side of a polyvinylidene chloride coated oriented polypropylene film and dried at an oven temperature of 210° F. The properties of the coated film were tested and the results of the testing, in comparison with a commercially available cold sealable adhesive, are summarized in Table 4.

                                      TABLE 4                                      __________________________________________________________________________     Results on Cold Seal Coatings                                                             Chemical Tg Crimp Seal (g/in)                                                                          Coating                                                                               Surface                              Adhesive   Composition                                                                             (°C.)                                                                      (H-Ct Wt)                                                                            (L-Ct Wt)                                                                            Uniformity                                                                            Tack                                 __________________________________________________________________________     Two-Stage Emulsion                                                                        EA/VAc/MAA                                                                              -0.4                                                                              578   438   Very good                                                                             Very Low                             polymerized polymer                                                            of the present                                                                 invention                                                                      Adhesive supplied                                                                         Polyisoprene                                                                            -1.6                                                                              671   165   Good to Fair                                                                          High                                 by Findley Adhesive,                                                           Inc.                                                                           __________________________________________________________________________      Base Film: PVDC Coated oriented polypropylene film                             Crimp: 80 psi, 1/2 sec., room temp.                                            HighCoating (HCt) Wt: 4.1 lb/3000                                              LowCoating (LCt) Wt: 0.99 lb/3000                                              EA = ethyl acrylate                                                            VAc = vinyl acetate                                                            MAA = methacrylic acid                                                   

Obviously, other modifications and variations of the present invention are possible in light of the above teachings. It is, therefore, to be understood that changes may be made in the particular embodiments of this invention which are within the full intended scope of the invention as defined by the appended claims 

We claim:
 1. A crosslinked, cold sealable, emulsion polymerized cohesive terpolymer for cold sealable packaging, comprising less than about 0.5% by weight of a crosslinking agent, the terpolymer is made from:(a) about 60 to about 70 percent by weight of a first monomer selected from the group consisting of ethyl acrylate, hexyl acrylate, isooctyl acrylate, butyl acrylate, methyl acrylate and vinylidene chloride; (b) about 25 to about 35 percent by weight of a second monomer selected from the group consisting of vinyl acetate, methyl methacrylate, styrene, ethyl methacrylate, and isobutyl methacrylate; and (c) about 1 to about 5 percent by weight of a third monomer selected from the group consisting of methacrylic acid, sulfoethyl methacrylate, acrylic acid, itaconic acid, and maleic acid, whereby said terpolymer has low tack characteristics, high crimp strength and is cohesive only to similarly coated substrates when cold sealed under pressure.
 2. The crosslinked, cold sealable, emulsion polymerized cohesive terpolymer of claim 1, having a weight average molecular weight of from about 20,000 to about 150,000 and a glass transition temperature in the range of from about -15° C. to about 15° C.
 3. The crosslinked, cold sealable, emulsion polymerized cohesive terpolymer of claim 2, wherein said first monomer comprises from about 60 to about 70 percent by weight of said terpolymer, said second monomer comprises from about 25 to about 35 percent by weight of said terpolymer and said third monomer comprises from about 1 to about 5 percent by weight of said terpolymer.
 4. The crosslinked, cold sealable, emulsion polymerized cohesive terpolymer of claim 3, wherein said first monomer is ethylacrylate, said second monomer is vinylacetate and said third monomer is methacrylic acid.
 5. The crosslinked, cold sealable, emulsion polymerized cohesive terpolymer of claim 4, having a weight average molecular weight of from about 50,000 to about 90,000 and a glass transition temperature of from about -10° C. to about 10° C.
 6. The crosslinked, cold sealable, emulsion polymerized cohesive terpolymer of claim 1, in which the crosslinking agent is ethylene glycol dimethylacrylate.
 7. The crosslinked, cold sealable, emulsion polymerized cohesive terpolymer of claim 1 in which the first monomer is selected from the group consisting of methyl acrylate, ethyl acrylate, butyl acrylate, hexyl acrylate and isooctyl acrylate.
 8. The crosslinked sealable, emulsion polymerized cohesive terpolymer of claim 1 in which the second monomer is selected from the group consisting of methyl methacrylate, ethyl methacrylate and isobutyl methacrylate.
 9. The crosslinked cold sealable, emulsion polymerized cohesive terpolymer of claim 1 in which the second monomer is vinyl acetate.
 10. The crosslinked cold sealable, emulsion polymerized cohesive terpolymer of claim 1 in which the third monomer is selected from the group consisting of methacrylic acid and acrylic acid. 